Dielectric fluids are often used in transformers, electrical switch gears, self-contained and pipe type cables and other pieces of equipment that require fluids that are generally fire and oxidation resistant and which include moderately good heat transfer characteristics and electrical properties. These dielectric fluids, however, are often limited in their use to, for example, equipment that is compatible with a more highly viscous fluid. These materials are not biodegradable and represent a potential environmental hazard if they leak or are accidentally spilled.
Moreover, these prior art dielectric fluids generally are not eligible for the "food grade" classification given by having USDA H1 approval and meeting the requirements under FDA regulation 21 CFR 178.3620(b) and having no PCB (poly chlorinated biphenyls), free benzene or polynuclear aromatics present.
Therefore it is desirable to develop and qualify a non-toxic biodegradable/-environmentally friendly dielectric fluid that would act as a direct replacement to these fluids. The new fluids must meet the rigid performance specifications of the current fluids (e.g. viscosity, color, water content, dielectric strength, and power factor) and must be able to operate over the temperature range of from about -50 to about 100.degree. C.
Some of the above inadequacies of the prior art dielectric fluids may be attributed to the fact that it was thought that a wide range of molecular weight species in the fluid was desirable. This conventional wisdom is exemplified in U.S. Pat. No. 4,284,522 (the '522 patent), which discloses a composition and method for forming a dielectric fluid composition wherein natural and synthetic hydrocarbons of different molecular weights are selectively blended to achieve a flat molecular weight distribution. According to the '522 patent, a wide molecular weight distribution improved the physical and chemical properties of the dielectric fluid. However, while a wide range of molecular weight compounds may have improved certain characteristics of the fluid, it also adversely affected various other physical and chemical parameters of the fluid in that, for example, it impeded the flow properties of the fluid composition.
In another disclosure of dielectric fluids, U.S. Pat. No. 4,082,866, it is taught that compounds having terminal olefinic bonds should be avoided. In U.S. Pat. No. 4,033,854 it was taught that a highly refined oil will not exhibit properties required of a dielectric fluid unless an aromatic hydrocarbon is added. Similarly, U.S. Pat. No. 4,072,620 taught the need for aromatic compounds to keep hydrogen gas absorbency at satisfactory levels which may be an indicator of corona resistance. The presence or addition of aromatics would not allow these materials to qualify as food grade.